Rub 1 teaspoon salt on to the mutton pieces. Set aside.
Heat oil in a stockpot. When hot, add chopped onions, and fry on medium heat until soft and translucent, about 7-10 minutes.
Add crushed ginger and garlic, and fry for roughly 30 seconds.
Add green chili (if using) and ground spices, and fry for roughly 1 minute until fragrant. Deglaze the pot with a splash of water if needed.
Add diced tomatoes, and the remainder of the salt. Fry tomatoes on high heat until soft, about 5-7 minutes.
Add chopped mutton, and continue to fry on high heat for 15-20 minutes.
Add roughly 3 cups water (just enough for the meat to be completely submerged) and cover with a tight fitting lid. Bring to simmer, and let the curry cook on medium-low heat for 1 ½ hour.
Lift cover and fold in diced spinach. Partially cover the stockpot, and let it simmer for another 30 minutes.
Check for salt, and adjust as needed. If there is still excess liquid, remove lid and increase heat to high to boil out some of the water.
Serve with roti, plain basmati rice or a vegetarian pulao of your choice.